Support _Float128 in tgmath.h

This patch adds tgmath.h support for _Float128, so eliminating the awkward caveat in NEWS about the type not being supported there. This does inevitably increase the size of macro expansions (which grows particularly fast when you have nested calls to tgmath.h macros), but only when _Float128 is supported and the declarations of _Float128 interfaces are visible; otherwise the expansions are unchanged.